1. Field of the Invention
The present invention relates to bone substitutes and, more particularly, to a shape memory polymer system for repairing bone defects.
2. Description of the Related Art
The nature of modern insurgency warfare combined with improved survival rates due to advances in body armor has created increasingly complex challenges for orthopedic reconstruction of extremity injuries in warfighters. More than 70% of military-related extremity fractures now involve massive bone loss, termed critically sized defects. Current grafting options for these defects have important limitations, described below. Additionally, for some combat-related critically sized defects there are no reliable treatments, and more than 7% of severe extremity injuries result in major amputation. There is a need for revolutionary functional materials that significantly improve control of graft integration (osseointegration) and bone formation (osteogenesis) during repair of critically sized defects.
To repair critically sized defects in the extremities of warfighters, live autograft—bone harvested from the same patient—remains the gold standard for small defects. But autologous bone is not always an option, particularly when multiple-limb trauma occurs within the same patient or when the defect is too large. Furthermore, complications include pain, infection, donor site morbidity, and inefficient repair due to limited osseointegration and remodeling. Allograft—bone harvested from a donor—provides the best current alternative, but often achieves limited osseointegration with a failure rate of 60% at 10 years. Synthetic bone graft substitutes have been developed and have generally been calcium phosphate or calcium sulfate space fillers or cements. Like allografts, synthetic bone graft substitutes lack living cells and function only as a scaffold for bone ingrowth (osteoconduction). Moreover, compared to auto- or allograft, synthetic grafts possess inferior mechanical strength and fracture resistance, preventing use in large defects that require rapid loadbearing capabilities. Graft approaches are often further complicated by damage to the periosteum—the tissue that covers the outer surface of bone and is critical to graft healing and remodeling.
In addition to the challenges listed above, information from major military medical institutions indicates that a subset of defects cannot be managed well with any existing treatment. Segmental defects—in which a segment of bone is missing and there is no continuity of bone within the fracture—are a particular problem. There is, therefore, an unmet need for highly effective bone substitutes. Ideally, these substitutes would conform to the defect, rapidly achieve mechanical properties similar to bone, integrate with neighboring bone, and support osteoconduction and remodeling.